Posted by: JeffMasters, 4:45 PM GMT on June 02, 2009
A near-average Atlantic hurricane season is on tap for 2009, according to the seasonal hurricane forecast issued June 2 by Dr. Phil Klotzbach and Dr. Bill Gray of Colorado State University (CSU). The CSU team is calling for 11 named storms, 5 hurricanes, and 2 intense hurricanes, and an Accumulated Cyclone Energy (ACE) 88% of average. Between 1950 - 2000, the average season had 10 named storms, 6 hurricanes, and 2 intense hurricanes. But since 1995, the beginning of an active hurricane period in the Atlantic, we've averaged 15 named storms, 8 hurricanes, and 4 intense hurricanes per year. The new forecast is a step down from their April forecast, which called for 12 named storms, 6 hurricanes, and 2 intense hurricanes. The new forecast calls for a near-average chance of a major hurricane hitting the U.S., both along the East Coast (28% chance, 31% chance is average) and the Gulf Coast (28% chance, 30% chance is average). The Caribbean is also forecast to have an average risk of a major hurricane.
The forecasters cited several reasons for an average season:
1) Sea surface temperature (SST) anomalies in the tropical Atlantic
are quite cool. In fact, these SST anomalies are at their coolest
level since July 1994. Cooler-than-normal waters provide less heat
energy for developing hurricanes. In addition, an anomalously cool
tropical Atlantic is typically associated with higher sea level
pressure values and stronger-than-normal trade winds, indicating a
more stable atmosphere with increased levels of vertical wind shear
detrimental for hurricanes. Substantial cooling began in November 2008
(Figure 1), primarily due to a stronger than average Bermuda-Azores
High that drove strong trade winds. These strong winds increased the
mixing of cool waters to the surface from below, and caused increased
evaporational cooling.
2) Hurricane activity in the Atlantic is lowest during El Niño years
and highest during La Niña or neutral years. This occurs because El
Niño conditions bring higher wind shear over the tropical Atlantic.
The CSU team expects the current neutral conditions may transition to
El Niño conditions (70% chance) by this year's hurricane season. I
discussed the possibility of a El Niño conditions developing this year
in a blog posted Friday
Read more on this subject and more at
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